This invention relates generally to waste heat recovery systems and, more particularly, to a organic rankine cycle system for extracting heat from a reciprocating engine.
Power generation systems that provide low cost energy with minimum environmental impact, and which can be readily integrated into the existing power grids or which can be quickly established as stand alone units, can be very useful in solving critical power needs. Reciprocating engines arc the most common and most technically mature of these distributed energy resources in the 0.5 to 5 MWe range. These engines can generate electricity at low cost with efficiencies of 25% to 40% using commonly available fuels such as gasoline, natural gas or diesel fuel. However, atmospheric emissions such as nitrous oxides (NOx) and particulates can be an issue with reciprocating engines. One way to improve the efficiency of combustion engines without increasing the output of emissions is to apply a bottoming cycle (i.e. an organic rankine cycle or ORC). Bottoming cycles use waste heat from such an engine and convert that thermal energy into electricity.
Most bottoming cycles applied to reciprocating engines extract only the waste heat released through the reciprocating engine exhaust. However, commercial engines reject a large percentage of their waste heat through intake after coolers, coolant jacket radiators, and oil coolers. Accordingly, it is desirable to apply an organic rankine bottoming cycle which is configured to efficiently recover the waste heat from several sources in the reciprocating engine system.
One problem that the applicants have recognized in such a system is that, if the organic rankine cycle (ORC) is disabled by component failure or for planned maintenance, the ORC working fluid will no longer be circulated through the reciprocating engine and the temperature of the ORC working fluid inside the engine as well as the critical engine components being cooled by this fluid will quickly exceed the safe level point of about 200° F., and it becomes then necessary to shut down the engine and cease operation.
A general concern with bottoming cycles is that of cavitation in the pump that circulates the working fluid. Such a system requires a pump with a relatively small flow rate (e.g. 18 lbm/s) and a large pressure rise (e.g. 250 psi). Optimum pump performance dictates a certain relationship between pump head (pressure differential), pump flow rate, and pump speed. For maximum efficiency, a small, high speed, radial pump is desirable. However, such a pump is subject to cavitation especially since it is downstream of the condenser where the liquid from the condenser is only slightly subcooled. Cavitation occurs when the liquid entering the pump starts to locally vaporize due to the initial flow acceleration. That is, since the higher local velocity results in a lower local pressure, vapor bubbles will be created if the local pressure is below the saturation pressure.
One approach to solving the cavitation problem is to use a less efficient regenerative pump, but this results in 35-45% efficiency rather than the 60-80% efficiency that is obtainable with radial pumps, which are more prone to cavitation.
It is therefore an object of the present invention to provide an improved ORC waste heat recovery system.
Another object of the present invention is the provision in an ORC system used to extract heat from a reciprocating engine, to allow continued operation of the engine when the ORC system is inactive.
Another object of the present invention is the provision in an ORC system for preventing cavitation of the pump.
Yet another object of the present invention is the provision in an ORC for prevention of pump cavitation while at the same time maintaining pump efficiency.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.